In the manufacture and preparation of ophthalmic lenses, it is common practice to fix a block of metal on one of the faces of a lens, thereby enabling the lens to be mounted on a lens holder in a surfacing machine, in an edging machine, or in a polishing or clear-polishing machine. This operation, which is referred to as "blocking", is normally performed by means of a blocking apparatus comprising a mold having an open mold cavity and an inlet orifice formed through a wall of the mold and opening out into the mold cavity, together with a tank containing a molten metal and including an outlet orifice for the molten metal.
In prior blocking apparatuses (see for example French patents numbers 1 50 454 and 2 465 562; and U.S. Pat. Nos. 3,049,766, 3,118,198, 3,237,349, 3,257,686, 3,468,366, 3,563,301, 4,025,033, and 4,136,727; and British patent number 1 297 037), the mold is conventionally filled with molten metal either by casting the metal into the mold under gravity or else by pumping the metal by means of a pump, generally a handactuated pump, or else by delivering the metal into the mold by putting the tank under pressure using compressed air. The flow of molten metal into the mold is usually controlled either by a cock or moving peg associated with an outlet orifice from the tank, or else by a valve placed in the pipe connecting the tank to the mold. In operation, such flow control devices are subject to breakdowns due to the fact that their moving parts are at least partially immersed in or in contact with molten metal and therefore operate under severe conditions. Further, during periods of inactivity, it may happen that the molten metal in the blocking apparatus solidifies in the vicinity of the moving parts or downstream therefrom, thus jamming the moving parts or creating an obstruction preventing any subsequent flow of molten metal to the mold.